1. Field of the Invention
The present invention concerns a measurement (data acquisition) sequence for three-dimensional magnetic resonance imaging suitable for use for production of magnetic resonance images that are optimally free of movement artifacts, as well as a magnetic resonance apparatus for this purpose.
2. Description of the Prior Art
Magnetic resonance (MR) technology is a known modality with which images of the inside of an examination subject can be obtained. The examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths from 0.2 Tesla to 7 Tesla and more) in an MR apparatus such that the nuclear spins of said examination subject become oriented along the basic magnetic field. To excite magnetic resonances, radio-frequency excitation pulses are radiated into the examination subject, and the excited magnetic resonances are measured and MR images are reconstructed based thereon. Rapidly-switched magnetic gradient fields are superimposed on the basic magnetic field for spatial coding of the measurement data. The acquired measurement data are digitized and stored as complex number values in a k-space matrix. By multi-dimensional Fourier transformation, an MR image can be reconstructed from the k-space matrix populated with these measured values.
Due to its relatively long measurement time, MR imaging is movement-sensitive, meaning that movement of the examination subject during the acquisition of the measurement data can lead to somewhat significant limitations in the image quality.
Therefore, various methods and/or measurement sequences exist with the goal to achieve a reduction of the sensitivity to movements of the examination subject so that an improved reconstruction of the image data is enabled.
Relatively complicated methods utilize external markers and superstructures with which movement in three dimensions in space can be detected and evaluated with optical means. However, such methods require additional hardware, and thus incur a high cost expenditure and are uncomfortable due to the necessary markings on the patient, so that such methods are typically used only to a limited extent.
Methods are also known using a special design of the measurement sequence that enables movement detection. For example, by a special design of the measurement sequence, an over sampling of a central region of k-space can ensue and the information obtained in this manner can be used for improved image reconstruction and for reduction of movement artifacts.
For example, in the acquisition of measurement data in the PROPELLER technique (also known as the BLADE technique) a k-space matrix is scanned (sampled) in segments, whereby the individual k-space segments are rotated relative to one another so that a central k-space region is scanned with each k-space segment. The over-sampling of the central k-space region enables a movement that occurs between the scanning of the individual k-space segments to be detected and to be taken into account in the image reconstruction. Other methods utilize spiral or radial k-space trajectories or an averaging of multiple, redundantly acquired measurement data, for example.
A disadvantage in this method is that the additional requirements necessary for over-sampling affect measurement time. Moreover, given non-Cartesian sampling, artifacts may accrue that originate from a non-optimal translation of the acquired measurement data to a Cartesian grid (“regridding”).
The methods described herein are typically tailored to the specific design of the employed measurement sequence and therefore allow a modification of the measurement sequence within narrow limits without degrading the implementation capability of the method. Many of the methods cannot be transferred to Cartesian scanning schemes without further measures.
Another method used in many cases for detection and/or for correction of movements occurred during the acquisition of the measurement data is the utilization of navigator signals, also called navigator echoes.
In this type of acquisition additional data (known as navigator signals) are acquired in addition to the actual measurement data with which the k-space matrix corresponding to the image to be produced is populated. These navigator signals allow movement of the examination subject acquired during the acquisition of the measurement data to be detected and, if necessary, to allow this to be taken into account in the reconstruction or the MR image or MR images so that movement artifacts are reduced.
A smaller region of the k-space matrix (for example one k-space row or a small central section of the k-space matrix) is typically scanned with navigator signal. A movement that may have occurred between the scanning of two navigator signals can be detected and/or taken into account in the image reconstruction by a comparison of the k-space values scanned by the navigator signal with regard to their amplitude and phase. Different types of navigator signals are known. Cloverleaf, orbital or spherical navigator signals are examples.
For a measurement sequence in which such navigator signals are acquired, the measurement duration of the measurement sequence and the subsequent image reconstruction sometimes increase significantly depending on the complexity of the navigator signals.
There is therefore a need to further develop measurement sequences that allow improvement of the image quality if and when movement of the examination subject occurs.